1. Field of the Invention
The present invention relates to a ball end mill having an arcuately shaped cutting edge at a forward end of a tool body, and in particular to an improvement of a ball end mill which has a cutting edge made of a composite sintered complex formed by integrally sintering cubic boron nitride (CBN), diamond or the like with cemented carbide.
2. Prior Art
FIGS. 5 and 6 depict a conventional ball end mill which comprises a generally cylindrical tool body 1 having an axis of rotation O therethrough and including a hemispherical forward end portion in which a pair of chip pockets 2 and 3 are formed in diagonally opposite relation to each other. Insert-receiving seats or recesses 4 and 5 are formed on those faces of the chip pockets which face in a direction of rotation of the tool body 1, and cutting inserts 6 and 7 are releasably attached to the insert-receiving seats 4 and 5, respectively.
Each of the inserts 6 and 7 is of a positive type, and comprises a plate-like insert body 8 defining a front face 9 and peripheral surfaces 10 extending from the front face 9. The intersection of one of the peripheral surfaces 10 with the front face 9 is of a quadrantal shape as viewed from the front face 9, and a composite sintered complex 11 is secured to this intersection to define a cutting edge 12. Each of the cutting inserts 6 and 7 is attached to the tool body 1 with the cutting edge 12 being protruded slightly radially outwardly from the hemispherical forward end portion of the tool body 1. Furthermore, the cutting edge 12 of the insert 6 is formed so as to be greater in length than the cutting edge 12 of the other insert 7, and is extended at its one end to a position adjacent to the axis O of rotation of the tool body 1.
The aforesaid composite sintered complex 11 is comprised of a two-layer construction having a base layer 14 of cemented carbide and an upper layer 13 of a sintered super hardened compact containing diamond, CBN or the like, and is secured by brazing or the like to the aforesaid intersection of the insert body 8 which is also formed of cemented carbide.
The ball end mill of the above construction has a high wear resistance due to the provision of the sintered super hardened compact layer, thereby exhibiting a relatively prolonged tool life.
In the cutting operation using a rotary cutter such as the above ball end mill, it is well known that the peripheral speed of the tool decreases radially inwardly of the tool body from an outer peripheral side, and hence the cutting resistance increases radially inwardly of the tool body. Accordingly, the cutting load exerted on the cutting edge during the cutting operation becomes the highest at a portion adjacent to the axis of the tool body, and hence the cutting edge 12, which is extended to the portion adjacent to the axis O of rotation of the tool body 1, is susceptible to chipping during the cutting operation.
For this reason, in the aforesaid ball end mill, that corner portion of the front face 9 of the cutting insert 6 which is to be disposed near the axis O of rotation of the tool body 1 is formed so as to be inclined or curved in such a manner that the radially inner portion of the cutting edge 12 extends radially inwardly of the tool body 1 in a direction opposite to the direction of rotation of the tool body 1, whereby the strength of the cutting edge 12 at the radially inner portion is enhanced to prevent the occurrence of chipping.
In order to define the inclined or curved surface at the corner portion of the front face 9 of the cutting insert 6, the corner portion will be simply removed. However, as will be seen from FIG. 6, when the corner portion is removed in the case of the cutting insert 6 which has the cutting edge 12 of composite sintered complex, not the sintered super hardened compact but the base layer 14 and the insert body 8 will be exposed. In order to prevent the exposure of these portions, the thickness of the upper layer of sintered super hardened compact of the composite sintered complex may be increased. However, this immediately leads to an increase of the manufacturing cost of the insert since the upper layer contains diamond or CBN. More specifically, when the distance between the axis O of the tool body 1 and the radially outer portion of the cutting edge 12 measured in the direction of thickness of the insert, i.e., the shifting distance h of the radially inner portion of the cutting edge 12 as indicated in FIG. 6, is set large, the radially inner portion of the cutting edge 12 is more reliably prevented from being chipped, However, since the thickness of the upper cutting edge layer 13 must be made large, a significant increase of the manufacturing cost is inevitable.
Furthermore, the base layer 14 and the insert body 8, which are both made of cemented carbide, have a lower wear resistance than the upper cutting edge layer 13 of sintered super hardened compact, and hence these base layer and insert body will be subjected to wear earlier than the upper cutting edge portion 13 of the cutting edge 12 after a prolonged period of use. As a result, the service life of the cutting insert may be shortened or may fluctuate.